The present invention generally relates to a wet process copier which electrostatically forms a latent image on a photoconductive element or similar image carrier and develops it by using a developer liquid composed of a carrier liquid and toner particles and, more particularly, to a method of fixing the developed image or toner image by heat.
In an electrophtographic copier, facsimile machine, printer or similar image forming apparatus, a latent image associated with image information is electrostatically formed on the surface of a photoconductive element which is a specific form of an image carrier. The latent image is developed by a developer liquid which is composed of a carrier liquid and toner particles dispersed in the carrier liquid. The developed image or toner image is transferred to a paper sheet or similar recording medium by electrophoresis and then fixed thereon by heat at a fixing station. Such a wet process uses toner particles the size of which is far smaller than toner particles used for a dry process. Specifically, the wet process is implemented by toner particles whose size is about 1 micron, while the dry process is implemented by toner particles whose size is about 10 microns. For this reason, the wet process is excellent in resolution and toner transparency and, therefore, produces clear-cut images. A fixing device for fixing the toner image as stated above has a heating roller in which a heater is accommodated, and a pressing roller which is pressed against the heating roller. While the paper sheet is transported through a nipping secion which is defined between the two rollers, the toner image on the paper sheet is heated under a predetermined pressure and thereby fixed on the paper sheet. The heating roller is often implemented by RTV (Room Temperature Vulcanized) silicone rubber or LTV (Low Temperature Vulcanized) silicone rubber.
On the other hand, in a fixing device for fixing a toner image produced by the dry process, a heating roller has to be durable and heat-resistant due to inherent operating conditions of the device, especially pressure and temperature. This kind of heating roller is, therefore, made of RTV or LTV silicone rubber to which is added a reinforcing agent or a filler such as silica or titanium dioxide (TiO.sub.2) or an antioxidant such as red oxide. The heating roller using such a mixture of rubber and additive, however, has poor separability and cannot be applied to the wet process fixing device which fixes a wet toner image. Specifically, a wet toner image contains about 70 weight percent to 80 weight percent of carrier liquid (Isopar) having small surface energy and is melted by heat with the toner particles and carrier liquid being mixed together. The separability of the heating roller and toner is especially low for such a reason. The heating roller made of additive-containing rubber and applied to the wet process is apt to cause a so-called offset phenomenon to occur.
To eliminate the offset phenomenon, a fixing system using a heat-bridging type toner has already been proposed, as disclosed in Japanese Patent Application No. 62-139426 by way of example. A heat-bridging type toner has an improved fluidity-to-temperature characteristic over ordinary toners and is an attempt to improve the fixing ability of the device. However, the heat-bridging type toner brings about a problem that microscopically the toner runs or spreads along the fibers of paper at the boundary between an image area where it is present and a background area where it is absent, depending on the kind of paper. A series of experiments showed that such a run or spread of toner along the fibers of paper is ascribable to the fact that the toner layer on a paper sheet is abruptly heated when nipped by the heating roller and pressing roller. Specifically, as the toner layer is abruptly heated, a mixture of toner components (pigment and resin) and a solvent (Isopar) forming the toner layer boils with the result that the toner whose viscosity has been lowered spreads along the fibers of paper. The lower the smoothness and the tightness of the paper, the more the spread of toner is aggravated.
Some different approaches may be contemplated to eliminate the spread of toner stated above, as follows:
(1) To lower the temperature of the heating roller; PA1 (2) To reduce the pressure acting between the heating and pressing rollers; PA1 (3) To reduce the concentration of Isopar in the toner layer to thereby raise the melting point of toner; and PA1 (4) To change the surface condition of the heating roller.
All the approaches mentioned above are not fully satisfactory, as determined by experiments. Specifically, the approach (1) degrades the fixing ability as to smearing and evenness of solid images. The approach (2) is also detrimental to the evenness of solid images even if a paper sheet having low smoothness is used. The approach (3) may be implemented by a blotter roller as already proposed, but a fixing device with such an implementation is complicated in construction. The approach (4) produces a undesirable pattern on the surface when a halftone image is reproduced.
The prior art fixing system using the heat-bridging type toner is not capable of reducing the offset phenomenon to a level having sufficient margins. We studied the offset phenomenon in varies ways and found that it greatly depends on the physical property (melting point) of toner derived from the fluidity characteristic. Specifically, although the melting point and other physical properties of a toner contained in a developer liquid is, of course, correlated with those of a dry developer, the solvent forming a wet toner image together with the toner as stated earlier serves to prevent the toner from depositing on the heating roller. It follows that the offset rarely occurs in a low temperature range of the heating roller, i.e., a so-called cold offset is hardly observed in such a range. However, as the temperature of the heating roller shifts to a high temperature range, a hot offset occurs due to the influence of the melting point of the toner layer. Presumably, the hot offset is ascribable to the viscosity of the toner layer and the physical depositing property of the toner layer on the heating roller.